Cure inhibited epoxy resin compositions and laminates prepared from the compositions

ABSTRACT

In summary, the invention is related to the discovery of a class of compounds which inhibits the reaction of a polyepoxide with a curing agent at low temperatures. More particularly the invention relates to an epoxy resin composition comprising 
     a) a polyepoxide; and 
     b) a cure inhibitor comprising boric acid or maleic acid. 
     In another embodiment, the invention comprises an epoxy resin composition comprising 
     a) a polyepoxide; 
     b) a cure inhibitor comprising boric acid or maleic acid, 
     c) a curing agent without phenolic hydroxyl moieties, and 
     d) optionally a catalyst for the reaction of the polyepoxide with the curing agent. 
     In still another embodiment the invention relates to a composition useful for curing a polyepoxide which comprises 
     a) a curing agent capable of reacting with a polyepoxide at elevated temperatures; and 
     b) an inhibiting amount of boric acid or maleic acid; and optionally 
     c) a catalytic amount of a catalyst useful for accelerating the reaction of a polyepoxide with the curing agent.

CROSS REFERENCE TO RELATED APPLICATION

This is a division from copending application Ser. No. 07/526,487 filedMay 21, 1990, abandoned.

BACKGROUND OF INVENTION

This invention relates to epoxy resin compositions containing compoundswhich inhibit the cure of the epoxy resins at lower temperatures. Theinvention further relates to compositions useful for curing epoxy resinswhich comprise curing agents for the epoxy resins and the inhibitordescribed above. The invention further relates to coatings prepared fromsuch inhibited resin compositions, and to materials coated with orencapsulated in such inhibited resin compositions. The invention furtherrelates to laminates, prepregs, encapsulated materials and compositesprepared from such inhibited resin compositions.

Epoxy resins are used in a variety of applications, as powder coatings,as electrical encapsulates, in composites, in solution coatings, inprepregs and in laminates. In many of these uses, it is desirable topartially cure the epoxy resin in a controllable manner, such that at alater time, when desired, the epoxy resin can be fully cured. In manyuses it is desirable to contact a curing agent for an epoxy resin withan epoxy resin in the presence of a catalyst for the reaction and havethe composition remain stable, that is uncured for a period of time. Atpresent it is quite difficult to control the curing of an epoxy resinand retain the stability of a composition comprising an epoxy resin, acuring agent and catalyst for the reaction of the epoxy resin with thecuring agent.

In some uses the epoxy resin, curing agent, and catalyst are contactedin solution and then applied as a coating on a substrate. In such uses,in order to achieve optimal properties in the eventually cured epoxyresin, the solvent or solvents must be removed. Often it is desirable toremove these solvents before significant curing takes place, otherwisethe solvent may be entrapped in the cured epoxy resin, thereby adverselyaffecting the final properties of the cured resin. Often, the solvent isremoved by exposing the coated article to elevated temperatures. At suchtemperatures the epoxy resin may have a tendency to begin to cure andentrap the solvent, thereby adversely affecting the final properties.

Cured and partially cured epoxy resins are used in laminates. A commonprocess used in preparing glass laminates involves passing a glass clothto be coated with an epoxy resin composition through a bath containingthe resin, a curing agent for the resin and a catalyst for the curingreaction. Generally the bath contains one or more organic solvents inwhich the various components are dissolved or dispersed, with a solidscontents of between about 45 and about 90 percent. The glass is residentin the bath for time sufficient to coat it with a mixture of the resin,curing agent and catalyst for the curing reaction. Thereafter the coatedglass is passed through a heated zone to remove remaining solvent byevaporation. Generally, this zone is at a temperature of between 120 and200° C. The residence time in this zone is sufficient to allow thesolvents to be completely volatilized away. Thereafter the glass cloth,coated with the resin, curing agent and catalyst from which the solventshave been removed, may be controllably partially cured by exposing it totemperatures at which curing occurs. Such a product is referred to as aprepreg. The prepregs may then be stacked or formed into a shape andexposed to conditions under which the curing agent and resin completesthe cure. In general, this involves contacting the various components ofa laminate at elevated temperatures under pressure, for a period of timesufficient for the epoxy resin to further cure. Usually the pressure isprovided by some kind of a press. Once the laminate is removed from thepress it may, optionally, be exposed to elevated temperature for aperiod of time to complete the curing reaction. In this curing processthe resin coating on the glass cloth flows and mixes with the coating onadjacent glass clothes thereby resulting in a fusing of the glass layerstogether, via the cured epoxy resin.

High glass transition temperatures are desirable for many uses,therefore methods of increasing the glass transition temperatures aredesired. It is further desired to significantly decrease the timenecessary to achieve complete cure. Prepreg producers and laminatorsdesire a composition which facilitates faster processing. This allowsthe more efficient utilization of processing equipment. In order tofacilitate faster processing the coated substrates must be exposed tohigher temperatures or the epoxy resin compositions must contain higherlevels of catalysts. Unfortunately, both solutions result in lesscontrol over the curing reaction and solvent can be trapped in the finalproduct thereby affecting the ultimate properties.

In one embodiment, a process referred to as continuous pressingtechnology is used. In this process the prepregs are contacted at muchhigher temperatures than in conventional laminate processing for shorterperiods, e.g. at temperatures around 210° C. for 2 to 4 minutes. It isvery difficult to achieve complete cure under such conditions. Onepotential solution to this is to add a larger amount of catalyst oraccelerator to the epoxy resin. Unfortunately if additional catalyst isadded in the coating bath, then the ability to remove the solventwithout significant curing of the epoxy resin, or to controllablypartially cure the epoxy resin, is significantly reduced.

What is needed is an epoxy resin composition that does not undergosignificant cure at temperatures at which solvents may be removed. Whatis further needed is an epoxy resin that can be controllably B-staged orpartially advanced. What is further needed is an epoxy resin compositionwhich allows the use of higher concentrations of curing catalysts oraccelerators, which are stable at temperatures at which solvent isremoved, and which allow controllable B-staging or partial advancement.What is further needed is an epoxy resin composition which when coatedon a substrate can be processed faster without hurting the ultimateproperties.

Frequently, for many products prepared using epoxy resins and curedepoxy resins several different entities may perform different parts ofthe manufacturing process. For example, one entity may make the resin, asecond entity may make the resin formulations used to impregnate thereinforcing material, and a third may make a prepreg, or other articleto be used. While a forth would make the final product such as alaminate or printed circuit board. Frequently the entity producing theprepreg or laminate has no expertise or desire to make the formulation.Therefore, it is desirable that a formulator be able to make acomposition useful in coating the materials to be laminated. The problemis that if the epoxy resin curing agent and catalyst are preformulated,the formulation may not have significant longterm storage stability.Under such circumstances the formulation may undergo curing andtherefore not be useful to the prepreg or laminate manufacturer. What isfurther needed is an epoxy resin composition containing a curing agentand accelerator for the cure which has significant stability at ambienttemperatures for several weeks.

SUMMARY OF INVENTION

In summary, the invention is related to the discovery of a class ofcompounds which inhibits the reaction of a polyepoxide with a curingagent in the presence of a catalyst for the curing reaction at lowtemperatures. More particularly, the invention relates to an epoxy resincomposition comprising

a) a polyepoxide and

b) a cure inhibitor comprising boric acid or maleic acid.

In another embodiment, the epoxy resin composition further comprises anepoxy resin catalyst, said catalyst being capable of accelerating thecure of the polyepoxide with a curing agent. In still anotherembodiment, the invention comprises an epoxy resin compositioncomprising

a) a polyepoxide;

b) a cure inhibitor comprising boric acid or maleic acid;

c) a curing agent for the polyepoxide which does not contain phenolichydroxyl moieties;

d) optionally, a catalyst for the reaction of the polyepoxide with thecuring agent; and

e) optionally, where a) is a diepoxide, a dihydroxy hydrocarbon orhalogenated dihydroxy hydrocarbon.

In still another embodiment the invention relates to a compositionuseful for curing a polyepoxide which comprises

a) a curing agent capable of reacting with a polyepoxide wherein thecuring agent does not contain phenolic hydroxyl moieties at elevatedtemperatures; and

b) an inhibiting amount of boric acid or maleic acid; and optionally

c) a catalytic amount of a catalyst useful for accelerating the reactionof a polyepoxide with the curing agent.

In still another embodiment, the invention relates to a process forcoating a reinforcing material with an epoxy resin compositioncomprising contacting a reinforcing material with an intimately mixedcomposition comprising:

a) a polyepoxide;

b) a curing agent for the polyepoxide, wherein the curing agent does notcontain phenolic hydroxyl moieties;

c) a cure inhibitor comprising boric acid or maleic acid;

d) a compound which catalyzes the curing of the polyepoxide with thecuring agent; and

e) optionally, where a) is a diepoxide, a dihydroxy hydrocarbon orhalogenated dihydroxy hydrocarbon.

The invention has several advantages. A composition comprising apolyepoxide, a curing agent, a catalyst, and inhibitor can be exposed totemperatures at which any solvent present is removed by evaporationwithout significant curing of the epoxy resin. Furthermore in thepresence of the inhibitor, the concentration of catalyst present may besignificantly higher without resulting in cure of the epoxy resincomposition during a solvent removal process. A higher crosslink densitymay be achieved by using a larger concentration of catalyst. The finalproducts prepared from these epoxy resin compositions may thereforeexhibit a higher glass transition temperature. Those resin compositionswhich contain a polyepoxide, curing agent, catalyst and inhibitor have asignificantly longer shelf life at ambient temperatures. Thecompositions of this invention may be processed at faster speeds as ahigher concentration of accelerator may be used in processing. Thecompositions may also be processed at high temperatures withoutadversely affecting the final product properties, for example in thecontinuous pressing process.

DETAILED DESCRIPTION OF INVENTION

This invention relates to novel resin compositions and articlescontaining such resin compositions or coated with such resincompositions. In one embodiment, the invention is a compositioncomprising a polyepoxide in the presence of a cure inhibitor comprisingboric acid or maleic acid. Such a composition could at some later timebe combined with curing agent which does not contain phenolic hydroxylmoieties and catalyst for the curing reaction and thereafter can be usedin one of many uses for an epoxy resin. Preferably this composition isstable for more than two months, more preferably six months or more, atambient temperatures.

The inhibitor and catalyst could be contacted with the polyepoxideseparately, or optionally, the inhibitor and catalyst could be contactedprior to contacting with the polyepoxide so as to form a complex, andthereafter the complex could be contacted with the polyepoxide.

In still another embodiment, a polyepoxide, a curing agent for thepolyepoxide, where the curing agent does not contain a phenolic hydroxylmoiety, and an inhibitor comprising boric acid or maleic acid could becontacted. The catalyst useful for reacting the polyepoxide with thecuring agent could be added to the composition and thereafter thecomposition could be used in any use of polyepoxide, known to thoseskilled in the art.

In still another embodiment, a polyepoxide could be contacted with acuring agent, an inhibitor comprising boric acid or maleic acid, and acatalyst for the reaction of an epoxy resin with the curing agent. Thecatalyst and inhibitor may be added separately, or added in the form ofa preformed complex. Preferably, such a composition is stable at ambienttemperatures for two weeks. Such a composition exhibits no significantchange in gel time at 150° C. and 170° C. over a period of 10 days, moreprefably 20 days and most preferably 30 days. This composition could beused in any of the known uses for polyepoxides i.e. epoxy resincompositions.

In another embodiment, the invention comprises a partially curedreaction product of a polyepoxide, a curing agent, wherein such partialcure occurs in the presence of catalyst for the reaction of the curingagent with a polyepoxide, and a cure inhibitor comprising boric acid ormaleic acid. Such partially cured product can thereafter be used inseveral applications, such as coatings, laminates, composites,encapsulants, by contacting with the appropriate substrate reinforcingmaterial or the like, and fully curing the composition. The processesfor obtaining such a partially cured product are well-known in the art.In particular, the components are contacted under condition such thatthe curing agent and polyepoxide undergo partial reaction. In someembodiments the reactive ingredients are partially cured on the surfaceof a substrate.

In another embodiment, the curing agent is contacted with a cureinhibitor comprising boric acid or maleic acid and, optionally, acatalyst for the cure of a polyepoxide by the curing agent. Thiscomposition is at some time thereafter contacted with polyepoxide andsubjected to curing conditions.

The compositions described herein may be found in various forms. Inparticular the various compositions described may be found in powderform, or alternatively in solution or dispersion. In those embodimentswhere the various compositions are in solution or dispersion, thevarious components of the composition may be separately dissolved in asolvent suitable for that component, then the various solutions arecombined and intimately mixed. Note the solvents for the variouscomponents are preferably chosen such that the solvents are misciblewith one another. Intimately mixed as used herein refers to agitating amixture of solutions which is a relatively homogeneous solution ordispersion. Alternatively, the components may be dissolved or dispersedin the same solvent or dispersant. In those embodiments wherein thecompositions are partially cured or advanced, the compositions of thisinvention may be found in a powder form, solution form, or coated on aparticular substrate.

Polyepoxide as used herein refers to a compound containing more than oneepoxy moiety. In another embodiment it refers to a mixture of compoundswhich contains, on average, more than one epoxy moiety per molecule.Polyepoxide as used herein includes partially advanced epoxy resins i.e.the reaction of a polyepoxide and a curing agent, wherein the reactionproduct has an average at least one unreacted epoxide unit per molecule.

Polyepoxides (polyglycidyl ethers of a polyhydroxy hydrocarbon) areprepared by reacting an epihalohydrin with a polyhydroxy hydrocarbon ora halogenated polyhydroxy hydrocarbon. The preparation of such compoundsis well known in the art. See Kirk-Othmer Encyclopedia of ChemicalTechnology 3rd Ed. Vol. 9 pp 267-289, incorporated herein by reference.

The epihalohydrins correspond to formula I ##STR1## wherein Y is ahalogen, preferably chloro or bromo, and most preferably chloro; and Ris hydrogen or C₁₋₄ alkyl, and more preferably methyl.

Polyhydroxy hydrocarbon means herein a compound with a hydrocarbonbackbone and more than one primary or secondary hydroxy moieties,preferably two or more. Halogenated polyhydroxy hydrocarbon means hereina compound with a hydrocarbon backbone which is substituted with one ormore halogens and more than one, preferably two or more, primary orsecondary hydroxy moieties. The hydroxyl moieties may be aromaticaliphatic or cycloaliphatic.

Among preferred classes of polyhydroxy hydrocarbons and halogenatedpolyhydroxy hydrocarbons are the bisphenols; halogenated bisphenols;hydrogenated bisphenols; novolac resins, i.e. the reaction product ofphenols and simple aldehydes, preferably formaldehyde; and polyalkyleneglycols.

Preferred polyhydroxy hydrocarbons, and halogenated polyhydroxyhydrocarbons include those corresponding to formulas 2 to 5: ##STR2##wherein: R¹ is separately in each occurrence C₁₋₁₀ alkylene, C₁₋₁₀haloalkylene, C₄₋₁₀ cycloalkylene, carbonyl, sulfonyl, sulfinyl, oxygen,sulfur, a direct bond or a moiety corresponding to the formula;

R² is separately in each occurrence C₁₋₃ alkyl or a halogen;

R³ is separately in each occurrence C₁₋₁₀ alkylene or C₅₋₅₀cycloalkylene; ##STR3## R⁴ is separately in each occurrence hydrogen,methyl, halomethyl, or ethyl, with the proviso that only one R⁴ on anethylene unit can be methyl, halomethyl or ethyl;

Q is separately in each occurrence a C₁₋₁₀ hydrocarbyl moiety;

Q' is separately in each occurrence hydrogen, cyano, or a C₁₋₁₄ alkylgroup;

m is independently in each occurrence an integer of 0 to 4;

m' is separately in each occurrence an integer of from 0 to 3;

p is a positive real number of 0 to 10;

q is a positive real number of 1 to 80.

R¹ is preferably C₁₋₃ alkylene, C₁₋₃ haloalkylene, carbonyl, sulfur, ora direct bond. R¹ is more preferably a direct bond, propylene, orfluorinated propylene (═C(CF₃)₂ --). R¹ is most preferably propylene. R²is preferably methyl, bromo or chloro; and most preferably methyl orbromo. R³ is preferably C₁₋₃ alkylene or polycyclic moiety correspondingto the formula ##STR4## wherein t is an average number between 1 and 6inclusive, preferably 1 and 3, most preferably 1. Preferably, m' is aninteger of 0 to 2. Preferably, m is an integer of 0 to 2. Preferably, pis a positive real number of 0 to 8; and more preferably 0 to 4.Preferably, q is a positive real number between 2 and 40, and morepreferably between 2 and 20 and most preferably 5 and 15. P and qrepresent an average number, as the compounds to which they refer aregenerally found as a mixture of compounds with a distribution of theunits to which p and q refer. Cycloalkylene as used herein refers tomono cyclic and poly cyclic hydrocarbon moieties.

Among preferred polyhydroxy hydrocarbon are the dihydroxy phenols.Preferable dihydroxy phenols include those which contain substituentsthat are non-reactive with the phenolic groups. Illustrative of suchphenols are 2,2-bis(3,5-dibromo-4-hydroxyphenyl) propane;2,2-bis(4-hydroxyphenyl) propane; 2,2-bis(3,5-dichloro-4-hydroxyphenyl)propane; bis (4-hydroxyphenyl) methane;1,1-bis(4-hydroxyphenyl)-1-phenyl ethane;1,1'-bis(2,6-dibromo-3,5-dimethyl-4 hydroxy phenyl) propane; bis(4-hydroxyphenyl) sulfone; bis (4-hydroxyphenyl) sulfide; resorcinol;hydroquinone; and the like. The preferred dihydroxy phenolic compound is2,2-bis(4-hydroxyphenyl) propane (bisphenol A) and2,2-bis(4-hydroxy-3,5-dibromophenyl) propane.

As used herein haloalkyl refers to a compound with a carbon chain andone or more of the hydrogens replaced with a halogen. Haloalkyl alsomeans compounds wherein all of the hydrogen atoms have been replaced byhalogen atoms. Alkylene as used herein refers to a divalent alkylmoiety.

The polyepoxides preferably correspond to one of formulas 6 to 9.##STR5## wherein R¹, R², R³, R⁴, m, m' and q are as defined previously;r is a positive real number of 0 to 40; and s is a positive real numberof 0 to 10. Preferably, r is a positive real number of 0 to 10, and mostpreferably 1 to 5. Preferably, s is a positive real number of 0 to 8;and most preferably 1 to 4. All of the variables referred to herein aspositive real numbers, i.e. r and s, are average numbers as thecompounds referred to contain a distribution of units.

In one embodiment polyepoxides refers to an advanced epoxy resin whichis the reaction product of one or more polyepoxides, as describedpreviously, with one or more polyhydroxy hydrocarbons or a halogenatedderivative thereof. Such polyhydroxy hydrocarbons have been describedpreviously. Alternatively, a polyepoxide can be reacted with a carboxylsubstituted hydrocarbon. A carboxyl substituted hydrocarbon is acompound with a hydrocarbon backbone and one or more carboxyl moieties,preferably more than one, and most preferably two. Preferably suchcompounds correspond to formula 10;

    R.sup.5 (COOH).sub.u                                       10

wherein R⁵ is C₁₋₄₀ hydrocarbyl moiety optionally containing oxygenalong the backbone,

and u is an integer of one or greater. R⁵ is preferably a C₁₋₄₀ straightor branched chain alkane or alkene, optionally containing oxygen.Preferably u is 1 to 4, and most preferably 2. Fatty acids and fattyacid dimers are among the useful carboxylic acid substitutedhydrocarbons. Included in the fatty acids are caproic acid, caprylicacid, capric acid, octanoic acid, versatic acid, decanoic acid, lauricacid, myristic acid, palmitic acid, stearic acid, palmitoleic acid,oleic acid, linoleic acid, linolenic acid, erucic acid, pentadecanoicacid, margaric acid, arachidic acid, and dimers thereof.

In one preferred embodiment a one or more polyhydroxy hydrocarbons orhalogenated derivatives thereof and one or more carboxyl substitutedhydrocarbons are reacted with the polyglycidyl ether of a polyhydroxycompound or halogenated derivative thereof. Procedures for performingsuch a reaction are well known in the art. See "The Handbook of EpoxyResins" by H. Lee and K. Neville (1967) McGraw Hill, N.Y. and U.S. Pat.Nos. 2,633,458; 3,477,990; 3,821,243; 3,907,719, 3,975,397; and4,071,477. Optionally, a small amount of a monohydroxy substitutedhydrocarbon can be included in the reaction mixture.

In another embodiment, the polyepoxide is the reaction product of apolyepoxide and a compound containing more than one isocyanate moiety, apolyisocyanate. Preferably the polyepoxide is an epoxy-terminatedpolyoxazolidone having from 5 to 30, preferably 5 to 20, most preferably10 to 20 weight percent isocyanate content and having from 50 to 100percent of the original isocyanate groups converted to oxazolidone ringsand from 0 to 50 percent of the original isocyanate groups converted toisocyanurate rings whenever prepared according to the process describedherein.

Epoxy-terminated polyoxazolidone (isocyanate modified epoxy resin) arepreferably prepared by the process which comprises reacting apolyepoxide compound with a polyisocyanate compound in the presence of acatalyst for the reaction of the epoxy and isocyanate groups at anelevated temperature. Preferably the process involves first adding (1)from 5 to 30, preferably 5 to 20, most preferably from 10 to 20, weightpercent of the polyisocyanate compound within a period of time of from 3to 90, preferably 15 to 60, most preferably 20 to 45, minutes to (2) amixture comprising (a) from 70 to 95, preferably 80 to 95, mostpreferably 80 to 90, weight percent of the polyepoxide compound and (b)from 0.01 to 2, preferably 0.02 to 1, most preferably 0.02 to 0.1,weight percent of the catalyst, said weight percentages being based onthe combined weight of the polyepoxide compound and polyisocyanatecompound. The mixture is thereafter heated to a temperature of from 110°C. to 200° C., preferably 120° C. to 180° C., most preferably 140° C. to160° C., for a period of time from 5 to 180, preferably 30 to 120, mostpreferably 60 to 90, minutes. The conversion of isocyanate groups to theoxazolidone and isocyanurate rings is controlled in any of the followingmanner:

(i) by increasing the reaction temperature at a given amount of thecatalyst and a given amount of the polyisocyanate compound used whenhigher conversion to oxazolidone rings is desired; or

(ii) by increasing the amount of the catalyst at a given reactiontemperature and a given amount of the polyisocyanate compound used whenhigher conversion to oxazolidone rings is desired; or

(iii) by increasing the amount of the polyisocyanate used at a givenamount of the catalyst and a given reaction temperature when higherconversion to isocyanurate rings is desired; or

(iv) by increasing the rate of addition of the polyisocyanate compoundat a given reaction temperature, given amount of the catalyst and givenamounts of the polyepoxide and polyisocyanate compounds when higherconversion to isocyanurate rings is desired.

The polyepoxides useful for this embodiment are described hereinbefore.The polyisocyanate compounds useful are represented by the followinggeneral formula:

    (O═C═N).sub.v --R.sup.6

wherein R⁶ is substituted or unsubstituted aliphatic, aromatic orheterocyclic polyvalent group and v has an average value of greater than1 to less than 5, preferably from 1.5 to 4, most preferably from 2 to 3.Examples of suitable polyisocyanates include 4,4'-methylenebis(phenylisocyanate) (MDI) and isomers thereof, higher functionalhomologs of MDI (commonly designated as "polymeric MDI"), toluenediisocyanate (TDI) such as 2,4-toluene diisocyanate and 2,6-toluenediisocyanate, m-xylylene diisocyanate, hexamethylene diisocyanate (HMDI)and isophoronediisocyanate. Mixtures of any two or more polyisocyanatescan also be used in the practice of the present invention. Othersuitable polyisocyanate compounds are described in U.S. Pat. Nos.3,313,747; 4,066,628 and 4,742,146.

Preferred polyisocyanate compounds are 4,4'-methylenebis(phenylisocyanate) (MDI) and isomers thereof, polymeric MDI andtoluene diisocyanate (TDI). The most preferred polyisocyanate compoundsare 4,4'-methylene bis(phenylisocyanate), isomers thereof and polymericMDI.

A suitable catalyst is employed to facilitate reaction of thepolyepoxide compound with the polyisocyanate compound. Examples ofuseful catalysts include zinc carboxylate, organozinc chelate compound,trialkyl aluminum, quaternary phosphonium and ammonium salts, tertiaryamines and imidazole compounds. The preferred catalysts are imidazolecompounds. Particularly, preferred catalysts are 2-phenyl imidazole2-methyl imidazole, 1-methyl imidazole, 2-ethyl-4-methyl imidazole and4,4'-methylene-bis(2-ethyl-4-methyl imidazole). The catalyst isgenerally employed in an amount of from 0.01 to 2; preferably 0.02 to 1,most preferably 0.02 to 0.1, weight percent based on the combined weightof the polyepoxide compound and polyisocyanate compound used. Thepolyisocyanate compound is employed in an amount of from 5 to 30,preferably 5 to 20, most preferably 10 to 20, weight percent, the weightpercent being based on the combined weight of the polyepoxide andpolyisocyanate reactants. The polyepoxide compound is employed in anamount of from 70 to 95, preferably 80 to 95, most preferably 80 to 90,weight percent, the weight percent being based on the combined weight ofthe polyepoxide and polyisocyanate reactants.

Preferably the catalyst is added to the reaction vessel containing thepolyepoxide prior to the start of the addition of polyisocyanatecompound. The catalyst can be dissolved in a suitable solvent prior tothe addition to the polyepoxide to improve homogenization if desired. Atemperature at which the catalyst is added is not critical. In generalthe catalyst is added at a temperature lower than the reactiontemperature. The temperature is then raised and the reaction temperaturemaintained while the controlled addition of the polyisocyanate to themixture of the catalyst and the polyepoxide is started. Thepolyisocyanate is added to the reaction vessel within a period of timeof from 3 to 90, preferably 15 to 60, most preferably 20 to 45, minuteswhile maintaining the reaction temperature. The reaction temperature ismaintained after the complete addition of the polyisocyanate for aperiod of time of from 5 to 180, preferably 15 to 120, most preferably30 to 90 minutes. In general, the reaction of the polyepoxide compoundand the polyisocyanate compound is conducted neat, i.e., in the absenceof a solvent or other liquid reaction diluent.

Curing agents useful in this invention are those compounds known to theskilled artisan to react with polyepoxides or advanced epoxy resins toform hardened final products and which do not contain phenolic hydroxymoieties which function to cure the epoxy resin. Included among suchuseful curing agents are materials which are acidic or alkaline.

Examples of suitable curing agents include among others, the polybasicacids and their anhydrides, such as, for example, the di-, tri-, andhigher carboxylic acids as oxalic acid, phthalic acid, terphtalic acid,succinic acid, alkyl and alkenyl-substituted succinic acids, tartaricacid, and particularly the polymerized unsaturated acids, such as forexample those containing at least 10 carbon atoms, and preferably morethan 14 carbon atoms, as for instance dodecenedioic acid,10,12-eicosadienedioic acid, and anhydrides such as phthalic anhydride,succinic anhydride, maleic anhydride, nadic anhydride(endo-cis-bicyclo-(2,21)-5 heptane 2,3 dicarboxylic anhydride), nadicmethyl anhydride (methyl bicyclo (2.21) heptene, 2,3 dicarboxylicanhydride isomers) ethylene diamine), pyromellitic anhydride,trimellitic anhydride and the like. Other types of acids that are usefulare those containing sulfur, nitrogen, phosphorus or halogens; chloridicacid, benzene phosphonic, sulfonyl dipropionic acidbis(4-carboxyphenyl)amide.

Other preferred curing agents include the amino-containing compounds,such as, for example, diethylene triamine, triethylene tetramine,dicyandiamide, benzoguanimine, melamine, pyridine, cyclohexylamine,benzyldimethylamine, benzylamine, diethylaniline, triethanolamine,piperidine, tetramethylpiperamine, N,N-dibutyl-1,3-propane diamine,N,N-diethyl-1,3-propane diamine, 1,2-diamino-2-methylpropane.2,3-diamino-2-methylbutane, 2,3-diamino-2-methylpentane,2,4-diamino-2,6-dimethyloctane, dibutylamine, dioctylamine,dinonylamine, distearylamide, diallylamine, dicyclohexylamine,methylethylamine, ethylcyclohexylamine, pyrrolidine,2-methylpyrrolidine, tetrahydropyridine, 2-methylpiperidine,2,6-dimethylpiperidine, diaminopyridine, meta-phenylene diamine and thelike, and soluble adducts of amines and polyepoxides and their salts,such as described in U.S. Pat. Nos. 2,651,589 and 2,640,037. Still otherexamples include the acetone soluble reaction products of polyamines andmonoepoxides, the acetone soluble reaction products of polyamines withunsaturated nitriles, such as acrylonitrile, imidazoline compounds asobtained by reacting monocarboyxlic acids with polyamines, sulfur and/orphosphorus-containing polyamines as obtained by reacting a mercaptan orphosphine containing active hydrogen with an epoxide halide to form ahalohydrin, dehydrochlorinating and then reacting the resulting productwith a polyamine, soluble reaction product of polyamines with acrylate,and many other types of reaction products of the amines.

Still other curing agents that may be used include boron trifluoride andcomplexes of boron trifluoride with amines, ethers, phenols and thelike. Friedel-Crafts metals salts, such as aluminum chloride, zincchloride, and other salts, such as zinc fluorborate, magnesiumperchlorate and zinc fluosilicate: inorganic acids and partial esters asphosphoric acid and partial esters as phosphoric acid and partial estersthereof including n-butyl orthothiophosphate, diethyl orthophosphate andhexaethyltetraphosphate and the like.

Another type curing agent to be employed includes the polyamidescontaining active amino and/or carboxyl groups, and preferably thosecontaining a plurality of amino hydrogen atoms. Examples of polybasicmaterial used in making these polyamides include, among others,1,10-decanedioic acid, 1,12-dodecanedienedioic acid,1,20-eicosadienedioic acid, 1,14-tetradecanedioic acid,1,18-octadecanedioic acid and dimerized and trimerized fatty acids asdescribed above. Amines used in making the polyamides include preferablythe aliphatic and cycloaliphatic polyamines as ethylene diamine,diethylene triamine, triethylene tetramine, tetraethylene pentamine,1,4-diamino-butane, 1,3-diaminobutane, hexamethylene diamine,3-(N-isopropylamino)propylamine and the like. Especially preferredpolyamides are those derived from the aliphatic polyamides containing nomore than 12 carbon atoms and polymeric fatty acids obtained bydimerizing and/or trimerizing ethylenically unsaturated fatty acidscontaining up to 25 carbon atoms. These preferred polyamides have aviscosity between 10 and 750 poises at 40° C., and preferably 20 to 250poises at 40° C. Preferred polyamides also have amine values of 50 to450.

Still another group of curing agents are those based on melaminereaction products containing methylol substituents.

Preferred are the polyamines and amides. Preferred curing agents includealiphatic polyamines, polyglycoldiamines, polyoxypropylene diamines,polyoxypropylenetriamines, amidoamines, imidazolines, reactivepolyamides, ketimines, araliphatic polyamines (i.e. xylylenediamine),cycloaliphatic amines (i.e. isphoronediamine or diaminocyclohexane)menthane diamine, 3,3-dimethyl-4,4-diamino-dicyclohexylmethane,heterocyclic amines (aminoethyl piperazine), aromatic polyamines,(methylene dianiline), diamino diphenyl sulfone, mannich base,phenalkamine, N,N'N"-tris(6-aminohexyl) melamine, and the like. Mostpreferred are cyanamide, dicyandiamide, and its derivatives,diaminodiphenyl sulphone and methylene dianiline.

Catalysts useful in this invention are those catalysts which catalyzethe reaction of a polyepoxide with a curing agent, and which remainslatent in the presence of the inhibitor at lower temperatures. Latentmeans herein that the catalyst does not catalyze the reaction at thelower temperatures. Preferably the catalyst is latent at temperatures of150° C. or below, and more preferably at 160° C. or below. Latency isdemonstrated by significantly higher stroke cure times when theinhibitor is present as compared to compositions not containing theinhibitor. Examples of preferred catalyst are compounds containingamine, phosphine, heterocyclic nitrogen, ammonium, phosphonium orsulfonium moieties. More preferred catalyst are the heterocyclicnitrogen and amine containing compounds and even more preferredcompounds are heterocyclic nitrogen containing compounds.

Examples of such heterocyclic nitrogen compounds include those describedin Copending, allowed patent application titled "Latent, Curable,Catalyzed Mixtures of Epoxy Containing and Phenolic Hydroxyl ContainingCompounds" U.S. Pat. No. 4,525,901 filed Nov. 18, 1988, relevant partsincorporated herein by reference. Among referred heterocyclic nitrogencontaining compounds useful as catalysts are those possessing in theheterocylic ring (1) a substituted C═N--C group and (2) a secondaryamino group, including the imidazoles, such as the substitutedimidazoles and benzimidazoles having the structural formula: ##STR6##respectively, wherein R⁷ is independently in each occurrence selectedfrom hydrogen atoms, halogen atoms, or an organic radical, such as ahydrocarbon radical or a substituted hydrocarbon radical, for example,the ester, ether, amide, imide, amino, halogen, or mercapto substitutedhydrocarbon radicals. Especially preferred imidazoles are those whereinR⁷ is hydrogen or hydrocarbon radical and preferably an alkyl, alkenyl,cycloalkyl, cycloalkenyl, aryl, alkaryl or arylalkyl radicals, andparticularly those containing no more than 15 carbon atoms.

A more detailed description of the chemistry of the imidazoles andbenzimidazoles including their properties and structural formulas isfound in the book by Klaus Hofmann entitled "Imidazole and ItsDerivates" published by Interscience Publishers, Inc., New York (1953).Examples of imidazoles include, among others, imidazole, benzimidazoleand substituted examples of suitable substituted imidazoles include:2-methyl imidazole; 2-ethyl, 4-methyl imidazole; 2-cyclohexyl, 4-methylimidazoles; 4-butyl, 5-ethyl imidazole; 2-butoxy, 4-allyl imidazole;2-carboethyoxybutyl, 4-methyl imidazole; 2-octyl 4-hexyl imidazole;2-methyl, 5-ethyl imidazole; 2-ethyl, 4-(2-ethylamino) imidazole;2-methyl, 4-mercaptoethyl imidazole; 2,5-chloro-4-ethyl imidazole; andmixtures thereof. Especially preferred are the alkyl-substitutedimidazoles; 2,5-chloro-4-ethyl imidazole; and mixtures thereof. Evenmore preferred are 2-methyl imidazole; 2-ethyl, 4-methyl imidazole; and2-phenyl imidazole. Especially preferred is 2-methyl imidazole.

Among preferred tertiary amines that may be used as catalysts are thosemono- or polyamines having an open chain or cyclic structure which haveall of the amine hydrogen replaced by suitable substituents, such ashydrocarbon radicals, and preferably aliphatic, cycloaliphatic oraromatic radicals. Examples of these amines include, among others,methyl diethanol amine, triethylamine, tributylamine, dimethylbenzylamine, triphenylamine, tricyclohexyl amine, pyridine, quinoline,and the like. Preferred amines are the trialkyl, tricycloalkyl andtriaryl amines, such as triethylamine, triphenylamine,tri(2,3-dimethylcyclohexyl)amine, and the alkyl dialkanol amines, suchas methyl diethanol amines and the trialkanolamines such astriethanolamine. Weak tertiary amines, e.g., amines that in aqueoussolutions give a pH less than 10, are particularly preferred.

Especially preferred tertiary amine accelerators are benzyldimethylamineand trisdimethylaminomethyl phenol.

The inhibitor is boric acid or maleic acid. Boric acid as used hereinrefers to boric acid or derivatives thereof, including metaboric acidand boric anhydride. It is believed that boric acid and its derivativesare all capable of functioning as an inhibitor when added to thecompositions of the invention either in the form added or in a convertedform. Boric acid is preferred over maleic acid. Inhibitor as used hereinshall mean a compound which functions to retard the curing reaction atlower temperatures, preferably, 140° C. or lower, more preferably 150°C. or lower. Inhibition is demonstrated by a longer stroke cure time(gel time) for compositions containing the inhibitor as compared to likecompositions not containing the inhibitor.

The inhibitor and catalysts may be separately added to the compositionsof this invention, or may be added as a complex. The complex is formedby contacting and intimately mixing a solution of the inhibitor with asolution of the catalyst. Such contacting generally is performed atambient temperature, although other temperatures may be used for exampletemperatures of about 0° C. to about 100° C., more preferably about 20°C. to about 60° C. The time of contacting is that sufficient to completeformation of the complex, and depends on the temperature used, with fromabout 5 to about 120 minutes preferred, and about 10 to about 60 minutesmore preferred. The solvents used for each component are preferablymiscible. Most preferably the same solvent is used. Preferred solventsfor the catalyst and the inhibitor are polar solvents, with alcoholsbeing preferred. Lower alcohols are even more preferred, with methanolmost preferred. Alternatively, the components of the complex may becontacted neat, in the absence of solvent and reacted as described.

The reinforcing material which may be coated with the compositions ofthis invention include any material which would be used by the skilledartisan in formation of composites, prepregs, laminates and the like.Examples of the forms of such materials are cloth, mesh, web, fibers, orthe like. Preferably, such materials are made from glass, fiberglass,paper, plastics such as aromatic polyamides, graphite and the like.Preferred materials include glass or fiberglass, in cloth or web form.Other additives may also be present including fillers, dyes, pigments,surfactants, flow control agents and the like.

In one embodiment the epoxy resin composition comprises a diepoxide, adihydroxyhydrocarbon or halogenated dihydroxyhydrocarbon, an epoxycuring agent which does not contain phenolic hydroxyl moieties, acatalyst for the epoxy curing reaction, and an inhibitor. In thisembodiment the diepoxide undergoes reaction with both thedihydroxyhydrocarbon or halogenated dihydroxy hydrocarbon and the curingagent. Thus the diepoxide is advanced in situ during the reaction. Inone embodiment, the diepoxide, dihydroxy hydrocarbon or halogenateddihydroxy hydrocarbon and inhibitor can be contacted prior to theaddition of the curing agent and catalyst. Those polyfunctionaldihydroxyhydrocarbon or halogenated dihydroxy hydrocarbon useful arewell-known in the art see for example Lee and Neville, supra; andBertram U.S. Pat. No. 4,594,291 col 8 lines 24 to 36, relevant partsincorporated herein by reference.

All concentrations of components used in this invention are quoted inparts of components per hundred parts of resin (phr). Such hundred partsof resin refers specifically to the polyepoxide included in thecomposition. The amount of curing agent which may be present may varydepending upon the particular curing agent used. Generally preferredamounts of curing agent are from about 0.5 to 30 parts of curing agentper hundred parts of resin. More preferably from 1.0 to 10.0, and mostpreferably from 2 to 4 parts. The catalysts are employed in a sufficientamount to result in a substantially complete cure of the epoxy resin,with some crosslinking. Preferably the catalyst is used in an amount offrom about 0.01 to 10 parts per hundred parts of resin, with from 0.05to 5.0 parts per hundred parts of resin being more preferred and from0.1 to 2.0 catalyst per hundred parts of resin being most preferred. Theamount of inhibitor used is that amount which significantly increasesthe stroke cure time (gel time) of a composition when compared to asimilar composition not containing the inhibitor. In one embodiment, theinhibitor is present in amounts up to about 2.0 phr, more preferably upto about 1.5 phr. Preferably about 0.05 phr or more of inhibitor isused, more preferably about 0.1 phr or more is used. In thoseembodiments where an equivalent amount of catalyst is used, or where acomplex of inhibitor and catalyst is prepared prior to contacting withthe polyepoxide, up to about 5.0 phr inhibitor may be used, provided thestroke cure rate is significantly increased. The Molar ratio ofinhibitor to catalyst is that ratio which is sufficient to inhibit thereaction of the polyepoxide or advanced epoxy resin as exhibited by anincrease in gel time as compared to a like composition free ofinhibitor. Simple experimentation can determine the particular levels ofinhibitor which will cause the aforementioned increase in gel time butstill allow a complete cure at elevated temperatures. A preferable Molarratio range of inhibitor to catalyst where up to 2.0 phr of boric acidis used, is from about 0.1:1.0 to about 4.0:1.0, with a more preferredrange being from 0.4:1.0 to about 3.0:1.0, with an even more preferredrange of 0.7:1.0 to about 2.0:1.0 with the most preferred range being0.9:1.0 to 1.5:1.0.

Compositions containing polyepoxide, curing agent, catalyst andinhibitor may be contacted with a reinforcing material in any methodknown to those skilled in the art. Examples of such contacting methodsinclude powder coating, spray coating, and contacting the reinforcingmaterial with a bath containing the composition. In a preferredembodiment the reinforcing material is contacted with the composition ina bath. Preferably the bath contains a solids levels of between about 50and 75% solids. In such a bath the various components of the epoxy resincompositions are dissolved or suspended in the bath. A single solventmay be used for the bath, but in many applications a separate solvent isused for each component added to the mixture. It is preferable that thevarious solvents used be miscible with one another. Such solvents ordiluents, include those which are volatile and escape from thecomposition prior to cure. Preferred solvents for the epoxy resins areketones, including acetone, methylethylketone and the like. Preferredsolvents for the curing agents are slightly polar solvents, amides, e.g.DMF, ether alcohols, e.g., methyl, ethyl, propyl or butyl ethers ofethylene glycol, diethylene glycol, propylene glycol or dipropyleneglycol for example, ethylene glycol monomethyl ether, 1 methoxy,2-propanol. The catalysts and inhibitors are preferably dissolved inpolar solvents, in particular alcohols, preferably lower alkanols andmost preferably methanol.

In one embodiment, the invention is a process for preparing prepregsfrom the resin composition and the reinforcing material. Such processcomprises contacting the reinforcing material with an intimately mixedbath comprising: a polyepoxide in a solvent; a curing agent for thepolyepoxide or advanced epoxy resin in a solvent; an inhibitorcomprising boric acid or maleic acid in a polar solvent; and a compoundwhich catalyzes the curing of the polyepoxide with the curing agent in apolar solvent. The coating occurs under condition such that thereinforcing material is coated with epoxy resin, curing agent,inhibitor, and catalyst. Thereafter the coated reinforcing materials arepassed through a heated zone at a temperature sufficient to cause thesolvents to evaporate, but below the temperature which the polyepoxideundergoes significant cure during the residence time in the heated zone.The reinforcing material preferably has a residence time in the bath ofbetween about 0.1 min and about 10 min, more preferably between about0.3 min and about 8 min, and most preferably between about 0.5 min andabout 3 min. The temperature of such bath is preferably between about 0°and about 100° C., more preferably between about 10° and about 40° C.and most preferably between about 15° and 30° C. The residence time ofthe coated reinforcing material in the heated zone is between about 0.5and about 15 min, more preferably between about 1 and about 10 min, andmost preferably between about 1.5 and about 5 min. The temperature ofsuch zone is sufficient to cause any solvents remaining to volatilizeaway yet not so high as to result in a complete curing of thecomponents. Preferable temperatures of such zone are between about 80°and about 230° C., more preferably between about 100° and about 200° C.,and most preferably between about 140° and about 190° C. Preferablythere is some means in the heated zone to remove the solvent, either bypassing an inert gas through the oven, or drawing a slight vacuum on theoven. In many embodiments the coated materials are exposed to zones ofincreasing temperature. The first zones are designed to cause thesolvent to volatilize so it can be removed. The later zones are designedto result in partial cure of the polyepoxide, i.e. so called B-staging.

In some embodiments the coated reinforcing material may be thentransported to another location for further processing, or alternativelythe material may be further processed immediately after solvent removal.In such further processing, several segments or parts of the coatedreinforcing material are brought in contact with one another.Thereafter, the contacted parts are exposed to elevated pressures andtemperatures sufficient to cause the epoxy resin to cure wherein theresin on adjacent parts react to form a continuous epoxy resin matrixbetween and about the reinforcing material. Before being cured the partsmay be cut and stacked or folded and stacked into a part of desiredshape and thickness. The pressures use can be anywhere from about 1 toabout 200 kg/cm², with from about 10 to about 100 kg/cm² beingpreferred. The temperature used to cure the resin in the parts orlaminates, depends upon the particular residence time, pressure used,and resin used. Preferred temperature which may be used are betweenabout 100° and about 190° C., more preferably between about 120° andabout 180° C., and most preferably between about 140° and about 175° C.The residence times may be anywhere from about 30 min to about 300 min,more preferably from about 45 to about 200 min, and most preferably fromabout 60 to about 180 min. One embodiment of such a process is known asa continuous process. In such process, the reinforcing material is takenfrom the oven and appropriately arranged into the desired shape andthickness and pressed at very high temperatures for short times, inparticular such high temperatures are from about 180° to about 250° C.,more preferably about 190° to about 210° C., at times of about 1 toabout 10 min and from about 2 to about 5 min. Such high speed pressingallows for the more efficient utilization of processing equipment. Insuch embodiments the preferred reinforcing material is a glass, web orwoven cloth.

In some embodiments it is desirable to subject the laminate or finalproduct to a post cure outside of the press. This step is designed tocomplete the curing reaction. The post cure is usually performed at fromabout 130° to about 200° C. for between about 20 and about 200 minutes.This post cure step may be performed in a vacuum to remove anycomponents which may volatilize.

The ultimate coated reinforced parts prepared from the composition ofthis invention often demonstrate a higher Tg than where the compositionsnot within the scope of this invention are used. In some embodiments theTg is at least 5° C. higher than parts prepared similarly usingconventional resins. More preferably the Tg is increased by at least 10°C. The parts prepared using the composition of this invention,demonstrate a higher solvent resistance, e.g., demonstrate a pick-up ofless than 1.0% N-methyl pyrrolidone in test methods conditions.

Furthermore such parts exhibit a higher thermal performance as there islittle or no solvent entrapped. The formulations of this inventionpreferably exhibit longer gel times at certain temperatures as comparedto prior art formulations.

SPECIFIC EMBODIMENTS

The following examples are presented to illustrate the invention and arenot intended to limit the scope of the claims. Unless otherwise statedall parts and percentages are by weight.

EXAMPLES EXAMPLE 1 Preparation of Varnish

A varnish is prepared by mixing and blending four solutions withagitation in a container for approximately 30 minutes. The foursolutions comprise 125 parts of a brominated epoxy resin having abromine content of 20% and an epoxy equivalent weight of 430, preparedfrom the diglycidyl ether of bisphenol A and tetrabromobisphenol A whichis dissolved in methylethylketone (MEK) with on 80% solids(non-volatiles) content; 2.19 parts of boric acid solution (20% inmethanol); 36 parts of a solution of dicyandiamide (7.5%) dissolved inmonomethylether of propylene glycol (69.9%) and dimethylformamide(22.6%); 4 parts of 2-methyl imidazole (2-MI) solution (10% inmethanol).

EXAMPLE 2 Preparation of Varnsih

A second varnish is prepared by mixing and blending the following foursolutions with simple agitation in a container for approximately 30minutes: 125 parts of a brominated epoxy resin having a bromine contentof 20% and an epoxy equivalent weight of 430, prepared from thediglycidyl ether of bisphenol A and tetrabromobisphenol A dissolved inmethylethylketone (MEK) to have 80% solids content; 1.65 parts of boricacid solution (20% in methanol); 40 parts of dicyandiamide (7.5%)dissolved in monomethyl ether of propylene glycol (69.9%) anddimethylformamide (22.6%); and 3.5 parts of 2-methyl imidazole (2-MI)solution (10% in methanol).

EXAMPLE 3 Preparation of Varnish

A solution containing 62.2 parts of a polyglycidylether of2,2-bis(4-hydroxy-phenyl) propane having an epoxide equivalent weight ofabout 180, 32.8 parts of tetrabromobisphenol A, and 5.0 parts ofmonomethyl ether of propylene glycol is prepared by blending and mixingthe polyglycidyl ether and tetrabromobisphenol A (TBBA) at roomtemperature and heating to 130° C. Approximately 30 min is required toobtain a homogenous mixture. The mixture is cooled to 70° C. and themonomethyl ether of propylene glycol is added. Another 30 minutes isrequired to obtain a homogenous solution.

A third varnish is prepared by mixing and blending the above describedsolution with the following three solutions with simple agitation in acontainer for approximately 30 minutes: 1.57 parts of boric acidsolution (20% in methanol); 34.2 parts of dicyandiamide (7.5%) dissolvedin monomethyl ether of propylene glycol (69.9%) and dimethylformamide(22.6%); and 3.8 parts of 2-methyl imidazole solution (10% in methanol)

EXAMPLE 4 Preparation of Varnish

A fourth varnish is prepared by blending and mixing the following foursolutions by simple agitation in a container for approximately 30minutes: 125 parts of a brominated epoxy resin having a bromine contentof 20% and an epoxy equivalent weight of 418, prepared from thediglycidyl ether of bisphenol A and tetrabromobisphenol A dissolved inmethylethylketone to have 80% solids content; 4.5 parts of a boric acidsolution (20% in methanol); 40 parts of dicyandiamide (7.5%) dissolvedin monomethyl ether of propylene glycol (69.9%) and dimethylformamide(22.6%); and 1.6 parts of a 2-methyl imidazole solution (50% inmethanol).

EXAMPLE 5 Preparation of Varnish--Not an Example of the Invention

A varnish is prepared by mixing and blending the following threesolutions by simple agitation in a container for approximately 30minutes: 125 parts of a brominated epoxy resin having a bromine contentof 20% and an epoxy equivalent weight of 430, prepared from thediglycidyl ether of bisphenol A and tetrabromobisphenol A which isdissolved in methylethylketone (MEK) with on 80% solids (non-volatiles)content; 40 parts of dicyandiamide (7.5%) dissolved in monomethyl etherof polypropylene glycol (69.9%) and dimethylformamide (22.6%); and 1part of 2-methyl imidazole in methanol (10% solution).

EXAMPLE 6 Preparation of Varnish--Not an Example of the Invention

A varnish is prepared by mixing and blending the following threesolutions by simple agitation in a container for approximately 30minutes: 125 parts of a brominated epoxy resin having a bromine contentof 20% and an epoxy equivalent weight of 430, prepared from thediglycidyl ether of bisphenol A and tetrabromobisphenol A which isdissolved in methylethylketone (MEK) with on 80% solids (non-volatiles)content; 40 parts of dicyandiamide (7.5%) dissolved in monomethyl etherof polypropylene glycol (69.9%) and dimethylformamide (22.6%); and 2.5parts of 2-methyl imidazole in methanol (10% solution).

EXAMPLE 7 Preparation of Varnish

A varnish is prepared by mixing and blending the following foursolutions by simple agitation in a container for approximately 30minutes: 125 parts of a brominated epoxy resin having a bromine contentof 20% and an epoxy equivalent weight of 430, prepared from thediglycidyl ether of bisphenol A and tetrabromobisphenol A dissolved inmethylethylketone (MEK), and having an 80% solids content; 40 parts ofdicyandiamide (7.5%) dissolved in monomethyl ether of propylene glycol(69.9%) and dimethylformamide (22.6%); and 1.67 parts of a solutionresulting from mixing 82.1 parts of 2-methyl imidazole and 63.65 partsof boric acid dissolved in 97.1 parts of methanol to give a 60% solidssolution.

EXAMPLE 8 Preparation of Varnish

A solution containing 62.2 parts of a polyglycidylether of2,2-bis(4-hydroxy-phenyl) propane having 32.8 parts oftetrabromobisphenol A, and 5.0 parts of acetone is prepared by blendingand mixing the polyglycidyl ether and tetrabromobisphenol A (TBBA) atroom temperature and heating to 130° C. Approximately 30 min is requiredto obtain a homogenous mixture. The mixture is cooled to 70° C. and theacetone is added. Another 30 minutes is required to obtain a homogenoussolution.

A varnish is prepared by mixing and blending the above describedsolution with the following three solutions with simple agitation in acontainer for approximately 30 minutes: 38 parts of dicyandiamide (7.5%)dissolved in monomethyl ether of propylene glycol (69.9%) anddimethylformamide (22.6%); and 1.425 parts of the product resulting frommixing 82.1 parts of 2-methyl imidazole and 63.65 parts of boric aciddissolved in 97.1 parts of methanol resulting in a solution having 60%solids.

EXAMPLE 9 Preparation of Varnish

A varnish is prepared by blending and mixing the following threesolutions by simple agitation in a container for approximately 30minutes; 125 parts of brominated epoxy resin having a bromine content of20% and an epoxy equivalent weight of 430, prepared from the diglycidylether of bisphenol A and a tetrabromobisphenol A dissolved inmethylethylketone (MEK) to have 80% solids content; 40 parts ofdicyandiamide (7.5%) dissolved in monomethyl ether of propylene glycol(69.9%) and dimethylformamide (22.6%); and 3.17 parts of the productresulting from mixing 82.1 parts of 2-methyl imidazole and 63.65 partsof boric acid dissolved in 97.1 parts of methanol to give 60% solids.

EXAMPLE 10 Preparation of Varnish--Not an Example of the Invention

A varnish is prepared by blending and mixing the following solutions bysimple agitation in a container for approximately 30 minutes; 58.95parts of the glycidyl polyether of 2,2-bis (4-hydroxy-phenyl) propanehaving an epoxide equivalent weight of about 180; 31.05 parts oftetrabromobisphenol A and 10 parts of methylethylketone; 40.8 parts ofdicyandiamide (7.5%) dissolved in monomethyl ether of propylene glycol(69.9%) and dimethylformamide (22.6%); and 5.4 parts of 2-ethyl,4-methyl imidazole dissolved in methanol (10% solution).

EXAMPLE 11 Preparation of Varnish--not an Example of the Invention

A laminating varnish is prepared by mixing and blending the followingsolutions by simple agitation in a container for approximately 30minutes at room temperature. 125 parts of brominated epoxy resin havinga bromine content of 21.5% and an epoxy equivalent weight of 520,prepared from the diglycidyl ether of bisphenol-A and atetrabromobisphenol A dissolved in methylethylketone (MEK) to have 80%solids content; 40 parts of dicyandiamide (7.5%) dissolved in monomethylether of polypropylene glycol (69.9%) and dimethylformamide (22.6%); and1 part of 2-methyl imidazole in methanol (10% solution).

Prepregs and Electrical Laminates Preparation Procedures A PrepregsPreparation

The varnishes of Examples 2 to 5 and 7 to 13 are applied to woven Eglass type 7628 available from INTER GLASS by pulling the cloth througha resin bath containing the laminating varnish and then up through adoctor bar set at 0.5 mm to 1.5 mm and then through staged heating zonesof an oven with temperatures varying from 140° C. to 190° C. forexamples 2 to 5 and from 150° C. to 170° C. for examples 7 to 13. Thetotal length of heating zone is 3 m. The glass cloth is passed throughthe resin bath and the heating zones at a rate of 0.8 to 3.2 m/min. Theparameters of the horizontal treater used are adjusted to produceprepregs with the following characteristics:

    ______________________________________                                                     Test Method                                                      ______________________________________                                        Rest gel time at 170° C.                                                              60-120   sec    NEMA LI 1-12.19                                Flow at 170° C.                                                                       2-25%           NEMA LI 1-14.32                                Volatiles      <1%                                                            Resin content  37-44%          NEMA LI 1-12.16                                ______________________________________                                    

NEMA refers herein to the National Electrical Manufacturers Association.The above described tests are performed according to the test procedurescorresponding to the numbers provided. Such test procedures areincorporated herein by reference. NEMA is located 2101 L. Street N.W.,Suite 300, Washington D.C. 20037.

Laminate Preparation

The laminates are obtained from 8 plies of prepreg (15 cm×15 cm) laid upbetween two sheets of copper foil and pressed in a laboratory press.

The prepregs obtained from varnishes examples 7 and 11 are pressed for15, 30, 60 and 90 min in the press at 165° C. The prepregs obtained fromexamples 9 and 10 are pressed at 210° C. for 2 and 3 mins. The pressureof the press is set at 40 kg/cm².

The prepregs obtained from varnishes from examples 2 to 4 are pressedfor 40 min in the press at 165° C. (10 min heating from room temperatureto 165° C. and 30 minutes at 165° C.) with controlled pressures toobtain laminate thickness of 1.3 to 1.6 mm. A prepreg prepared using avarnish from example 4 is pressed at 200° C. for 5 minutes at 30 kg/cm².

Test Methods Varnish Properties Measurements

Gel Time Measurement

The gel time of different varnishes is measured by reacting the mixtureon the surface of a hot plate at 150° C., 160° C. and 170° C. Thetemperature is calibrated by Signotherm temperature indicators.

Glass Transition Temperature Measurements

The glass transition temperature is measured on film prepared from thevarnish formulation used for gel time measurement. The films are curedon a hot plate 170° C. for 30 min. Samples are run at 10° C./min. Theglass transition temperature is measured by a Mettler TA 2000 DTA at aheating rate of 10° C./min. The inflection point minus 5° C. of theendothermic response was taken as the glass transition temperature.

Prepreg Properties Measurements

Gel Time Measurements--Stroke Cure

The partially cured resin is removed from the glass, and put onaccurately regulated hot plate at 170° C. It is then stroked with ametal wire until gelation occurs. The time from the first contact of theresin dust with hot plate to gelation is recorded as the gel time orrest reactivity of the prepreg.

Volatile Determination

A sample of prepreg approximately 10 cm×10 cm is weighed to the nearest0.001 gram (W₁).

The sample is placed in a circulating air oven at 163° C. for 15 minutesand on removal it is placed immediately in a desicator to cool. Whencool, the sample is reweighed and the weight recorded as W₂. Thevolatile content is calculated from: ##EQU1##

Resin Content

The resin content is measured by simply weighing an exactly determinedsurface of prepreg, and comparing with the weight of the same area ofglass cloth. The resin content is given by the: ##EQU2##

Flow Measurement

The flow test indicates the extent of the B-staging and it simulates thebehaviour of the resin system within the pressing operation. Accordingto NEMA LI 1-14.32 procedure six plies of prepreg of determined size areweighed to the nearest 0.01 gram, stacked together with their edges incareful alignment, fastened together and loaded into the preheatedpress. The temperature of the press plates is 170° C. and pressure is 14kg/cm². After 10 minutes the specimen is removed and allowed to cool.Then the flowed resin is removed by cutting and weighing to the nearest0.01 gram. The resin flow is calculated from the equation: ##EQU3##

Flow Measurement

Prepregs obtained from continous pressing formulation

Six samples, 10 cm×10 cm are cut in a prepreg sheet and stuck betweentwo copper foils, the shiny copper side facing the prepreg and acting asrelease sheets. The lab press is heated between 210° C., with a pressureof 30 bars. Pressing time of 1 minute obtains a flow sample. The sampleis weighed and a 50 cm² disk is punched out of the 100 cm² sample. Flowis given as: ##EQU4##

Laminate Testing

Chemical Resistance

To measure the chemical resistance, pieces of etched laminates 5 cm×5 cmare cut, weighed and dipped for 30 minutes in N-methylpyrrolidonesolvent at 23° C. The pieces are dried and then weighed; the pick-up istaken as the gain in weight.

Glass Transition Temperature Measurements

The glass transition temperature of laminates is measured by a MettlerTA 2000 DTA at a heating rate of 10° C./min. The inflection point minus5° C. of the endothermic response was taken as the glass transitiontemperature.

Moisture Resistance

Twelve pieces of etched laminates 6 cm×1.5 cm are placed in a pressurecooker during 30, 60, or 90 minutes at 120° C. under 1.2 atm. steam.Then, after being dried are dipped (20 seconds) in a tin solder bath(260° C.). The moisture absorption is determined by observation from theformation of blisters.

Copper Peel Strength

The binding strength between the copper and the epoxy resin isdetermined through the strength needed to peel the copper off alaminate. The copper peel strength test is performed according to IPCtest Method 2.4.8 Peel Strength, Metal Fort. A 10 cm×10 cm square ofcopper foil is laminated to a laminate of the invention. The specimensare tested for peel strength at room temperature. The copper foil ispeeled back approximately one inch, so the line of peel is perpendicularto the edge of the specimen. The specimen is clamped on a horizontalsurface with the peeled metal strip projecting upward for 1 inch. Theend of the foil is gripped between the jaws of the clamp such that thejaws cover the full width of the metal strip and are parallel to theline of peel. Force is exerted in the vertical plane, and the metal foilis pulled at 2 inches per minute. The load is observed and converted topounds per inch of width. The final peel strength is the average ofthree tests.

The gel times of varnishes from Examples 1 to 6 are measured. Theresults are compiled in Table I.

                  TABLE I                                                         ______________________________________                                        GEL TIMES OF DIFFERENT                                                        VARNISHES AT DIFFERENT TEMPERATURES                                                   Gel Time at                                                                              Gel time at                                                                              Gel time at                                     Examples                                                                              150° C. (sec)                                                                     160° C. (sec)                                                                     170° C. (sec)                                                                   Tg*                                    ______________________________________                                        1       445        319        203      150                                    2       390        271        172      148                                    3       310        220        140      148                                    4       353        240        161      152                                     5**    362        245        171      125                                     6**    194        133         88      134                                    ______________________________________                                         *The glass transitions are measured on films obtained by curing the           different varnishes at 170° C. for 30 min.                             **Not an example of the invention.                                       

                  TABLE II                                                        ______________________________________                                        FR-4 RESIN FORMULATIONS, VARNISHES,                                           PREPREG AND LAMINATES PROPERTIES                                              ______________________________________                                                           4**                                                        Example      2       3       I     II    5                                    ______________________________________                                        TREATER                                                                       SETTING                                                                       Oven temp. zones                                                                           148/    150/    150/  154/  150/                                 1/2 (°C.)                                                                           183     190     171   182   170                                  Winding speed                                                                              1.5     3       0.85  2     2.6                                  (m/min)                                                                       VARNISH                                                                       PROPERTIES                                                                    Gel time 170° C. (sec)                                                              172     140     161   161   171                                  Viscosity (Ford Cup                                                                        28      27      40    40    44                                   No. 4, sec)                                                                   PREPREG                                                                       PROPERTIES                                                                    % Resin content                                                                            40.7    39      46.5  44    37.5                                 % Volatiles  0.2     0.2     0.2   0.2   0.2                                  Rest gel time,                                                                             66      54      --    65    140                                  170° C. (sec)                                                          % Flow       11.2    13.8    6.6   18    25.7                                 Appearance   good    excell. good  good  good                                 LAMINATE                                                                      PROPERTIES                                                                    Tg (°C.)                                                                            143     142     148   152   127*                                 Chemical     0.6     0.2     0.4   0.3   0.5*                                 Resistance (% NMP                                                             pick-up)                                                                      Blister Resistance                                                                         60      60      --    --     60*                                 (PCT MIN)                                                                     Copper Peel  18      18      17    18    21*                                  Strength (N/cm)                                                               ______________________________________                                         *Laminates properties from the following press cycle                     

                          Pressure  Time                                                                (kg/cm.sup.2)                                                                          (min)                                          ______________________________________                                        1. Room temperature to 170° C.                                                               14       30                                             2. Keep at 170° C.                                                                           40       90                                             3. Cool to room temperature                                                                         40       15                                             ______________________________________                                         **The same varnish 4 is used to prepare continuous press prepregs (I) and     standard FR4 prepregs (II) at different treating speeds. Prepregs (I) are     pressed at 200° C. for 5 minutes to obtain laminates for testing. 

    Example        7          8       11*                                         ______________________________________                                        VARNISH                                                                       PROPERTIES                                                                    Gel time 170° C. (sec)                                                                147        119     300                                         Viscosity (Ford Cup                                                                          29         20      21                                          No 4, sec)                                                                    PREPREG                                                                       PROPERTIES                                                                    % Resin content                                                                              43         41      40                                          % Volatiles    0.17       0.26    --                                          Rest gel time, 60         104     139                                         170° C. (sec)                                                          % Flow         14.3       21.6    11.8                                        Appearance     good       good    good                                        LAMINATE                                                                      PROPERTIES                                                                    (obtained from 30                                                             min at 165° C.)                                                        Tg (deg C.)    142        142     122                                         Chemical       0.2        0.2     3.3                                         resistance (% NMP                                                             pick-up)                                                                      Blister resistance                                                                           30         30      30                                          (PCT MIN)                                                                     Copper peel    17.2       17      20                                          strength (N/cm)                                                               ______________________________________                                         *Not an example of the invention                                         

Laminates are prepared from Examples 9, 10. The properties of thevarnishes, prepreg and laminates are compiled in Table III.

                  TABLE III                                                       ______________________________________                                        FR-4 RESIN FORMULATIONS FOR CONTINUOUS                                        PRESSING TECHNOLOGY-CURED RESINS, PREPREGS                                    AND LAMINATES PROPERTIES                                                      Example             9        10                                               ______________________________________                                        VARNISH PROPERTIES                                                            Gel time 170° C. (sec)                                                                     91       106                                              Viscosity (Ford Cup No 4, sec)                                                                    28       19                                               PREPREG PROPERTIES                                                            % Resin content     40.7     42                                               % Volatiles         0.1      0.1                                              % Flow              4.5      3.5                                              Appearance          good     good                                             LAMINATE PROPERTIES                                                           (obtained from 2 and 3 min                                                    at 210° C.)                                                            Tg (deg C. I/II)*                                                             2 min               145/145  136/136                                          3 min               146/147  136/135                                          Chemical resistance                                                           (% NMP pick-up)                                                               2 min               0.1      0.5                                              3 min               0.05     0.5                                              Copper peel strength (N/cm)                                                                       17       18                                               ______________________________________                                         *The Tg is determined twice for the same sample and both values are           reported. The second determination is made after the sample is cooled fro     the previous test.                                                       

The glass transition and solvent resistance of laminates from varnishesof Example 7 and 11 are measured. Results compiled in Table IV.

                  TABLE IV                                                        ______________________________________                                        GLASS TRANSITION TEMPERATURE AND SOLVENT                                      RESISTANCE MEASURED FROM LAMINATES OF                                         EXAMPLES 7 AND 11.                                                            Press Time (min                                                                              15      30       60    90                                      ______________________________________                                         7   Tg (°C.)                                                                             125     142    145   146                                        % NMP pick-up 0.3     0.16   0.16  0.15                                  11*  Tg (°C.)                                                                             107     122    123   124                                        % NMP pick-up 4.4     3.3    2.5   2.4                                   ______________________________________                                         *Not an example of the invention.                                        

The top number is the glass transition temperature. The bottom number isthe percent weight gain when exposed to N-methyl pyrolidone (NMP).

                                      TABLE V                                     __________________________________________________________________________    COMPARISON OF TREATER PARAMETERS AND                                          PREPREG PROPERTIES FOR EXAMPLES 7, 8 AND 5                                    Examples     7       8       5*                                               __________________________________________________________________________    VARNISH GEL TIME                                                                           147     119     171                                              (170° C., sec)                                                         OVEN TEMP.                                                                    (recorded)                                                                    Zone 1 (deg C.)                                                                            150     145     150                                              Zone 2 (deg C.)                                                                            170     165     170                                              GAP (mm)     0.6 0.6 1   1   0.75                                                                              0.65                                         WINDING SPEED                                                                              2.5 2.8 2.6 3   2.3 2.6                                          (m/min)                                                                       PREPREG PROP.                                                                              39.8                                                                              42.1                                                                              41.2                                                                              43.0                                                                              39.5                                                                              37.5                                         Resin content (%)                                                             Rest gel time at                                                                           102 126 104 108 116 140                                          170° C. (sec)                                                          Flow (%)     16.7                                                                              18.3                                                                              21.6                                                                              23  17.6                                                                              25.7                                         Appearance   good                                                                              good                                                                              good                                                                              good                                                                              good                                                                              good                                         __________________________________________________________________________     *Not an Example of the Invention.                                        

EXAMPLE 12

An isocyanate modified epoxy resin is prepared by the process describedbelow from the following components; 400 parts of a polyglycidyl etherof 2,2-bis(4-hydroxy-phenyl) propane (bisphenol A) having an epoxideequivalent weight of about 180; 260 parts of brominated epoxy resin withbromine content of 49% prepared from the diglycidyl ether of2,2-bis(4-hydroxy-3,5 dibromo-phenyl) propane (tetrabromobisphenol A)and 2.2-bis(4-hydroxy-3,5-dibromo pheynl) propane (tetrabromobisphenolA); 0.6 parts 2-phenyl imidazole dissolved in an equal amount ofmethanol; 90 parts methylene diphenyl diisocyanate (available from TheDow Chemical Company under the trademark Voranate* M 220) and 250 partsof methylethylketone. The resin is prepared by the following procedure.

The liquid epoxy resin and brominated epoxy resin are charged to areactor. The mixture is stirred and heated under a nitrogen atmosphereto about 90° C. The catalyst of 2-phenyl imidazole is added to themixture once it reaches 90° C. The mixture is heated to 150° C.Methylenediphenyl diisocyanate is added over a period of 30 to 45minutes. The reaction temperature is maintained between 155° and 175° C.during the addition. The mixture is heated on addition 30 minutes. Aportion is removed and titrated to determine the epoxy content. It isdetermined to be 12%. The mixture is cooled to 130° C. andmethylethylketone is added.

133.33 parts of the isocyanate modified epoxy resin solution is blendedwith 2.5 parts of a boric acid in methanol solution (20%) (0.5 phr boricacid); 42.66 parts of dicyandiamide (7.5%) in monomethyl ether ofpropylene glycol (69.9%) and dimethylformamide (22.6%) solution, (3.2phr); and a 1.1 parts of 50 percent 2-methyl imidazole in methanol (0.55phr), at room temperature for 30 minutes.

EXAMPLE 13 Not an Example of the invention

133.33 parts of the isocyanate modified epoxy resin prepared as inExample 12 are blended with, 42.66 parts of a dicyandiamide (7.5%) inmonethylether of propylene glycol (69.9%) and dimethylformamide (22.6%)solution and 0.18 parts of 50% 2-methyl imidazole in methanol (0.09 phr)at room temperature for 30 minutes.

Prepregs and laminates are prepared using the varnishes of Examples 12and 13. The properties of the varnishes, prepregs, and laminates aredetermined by the processes described above. The results are compiled inTable VI.

                  TABLE VI                                                        ______________________________________                                                        Example 12                                                                             Example 13*                                          ______________________________________                                        VARNISH PROPERTIES                                                            Gel time (170° C.) sec                                                                   191        253                                              Viscosity (Ford cup No. 4, sec)                                                                 28         30                                               Treating parameter                                                                              154/174    150/178                                          Oven temp. (Zone 1/2  , °C.)                                           Winding speed (m/min)                                                                           2.1        1.8                                              PREPREG PROPERTIES                                                            Resin Content (%) 44         43                                               Gel Time (170° C., sec)                                                                  70         115                                              Flow (%)          16.5       --                                               LAMINATE PROPERTIES                                                           (45 min at 175° C.)                                                    Tg (°C. I/II)**                                                                          179/182    155/165                                          NMP pick-up (%)   0.11       0.18                                             ______________________________________                                         *Not an example of the invention                                              **The Tg is determined twice for the same sample and both values are          reported. The second determination is made after the sample is cooled fro     the previous test.                                                       

EXAMPLE 14 to 19

Varnishes are prepared by blending an 125 parts of a brominated epoxyresin having a bromine content of 20% and an epoxy equivalent weight of418, prepared from the diglycidyl ether of bisphenol A andtetrabromobisphenol A dissolved in methylethylketone (80% solids); 40parts of a solution of dicyandiamide (7.5%) in monomethyl ether ofpropylene glycol (69.9%) and dimethylformamide (22.6%), (3 phr); acatalyst and in examples 15, 17 and 19 boric acid. The relative amountsof catalyst and boric acid are compiled in Table VIII. The gel time andglass transition temperature are measured of films cured at 170° C. for30, and 60 minutes prepared from the varnishes. The results are compiledin Table VII.

                                      TABLE VII                                   __________________________________________________________________________                                     Glass                                                         Boric-          Transition                                                    acid                                                                              Varnish     °C.                                   Example                                                                             Catalyst                                                                              phr                                                                              phr 150° C.                                                                    160° C.                                                                    170° C.                                                                    30 min                                                                            60 min                                   __________________________________________________________________________     14*  Ethyltriphenyl                                                                        0.2                                                                              0   --  --  307 --  131                                            phosphonium                                                                   acetate.Acetic                                                                acid complex                                                            15    Ethyltriphenyl                                                                        1.0                                                                              0.524                                                                             --  --  391 --  140                                            phosphonium                                                                   acetate.Acetic                                                                acid complex                                                             16*  butyl   0.2                                                                              0   366 299 240 130 138                                            dimethyl                                                                      amine                                                                   17    butyl   1.0                                                                              0.524                                                                             --  347 245 136 143                                            dimethyl                                                                      amine                                                                    18*  Diisobutyl                                                                            0.2                                                                              --  --  --  510 --  --                                             amine                                                                   19    Diisobutyl                                                                            1.0                                                                              0.524                                                                             --  --  >780                                                                              --  --                                             amine                                                                   __________________________________________________________________________     *Not an example of the invention                                         

EXAMPLES 20-40

Varnishes are prepared from three different resins using varying amountsof curing agent, dicyandiamide; inhibitor, boric acid; and catalyst,2-methyl imidazole. Resin A is brominated epoxy resin having a brominecontent of 20% and an epoxy equivalent weight of 418, prepared from thediglycidyl ether of bisphenol A and tetrabromobisphenol A dissolved inmethylethylketone (80% solids).

Resin B is brominated epoxy resin with a bromine content of 19 percentand an epoxy equivalent weight of 395 prepared from the diglycidyl etherof bisphenol A and tetrabromobisphenol A in methylethylketone 80%solids.

Resin C is a brominated epoxy resin having a bromine content of 20% andan epoxy equivalent weight of 430, prepared from the diglycidyl ether ofbisphenol A and tetrabromobisphenol A dissolved in methylethylketone(80% solids content).

The resin solutions are blended with dicyandiamide (7.5%) in monomethylether of propylene glycol (69.9%) and dimethylformamide (22.6%); boricacid in methanol (20% boric acid); and 2-methyl imidazole in methanol(10% 2-methyl imidazole); at ambient temperature with agitation forabout 30 minutes. The gel time of each varnish is determined at 150°,160° and 170° C. A film is prepared from each resin, which is cured for30 minutes at 170° C., the films from examples 33 to 38 are cured for 60minutes. The glass transition temperature of the films are determined.The results are compiled in Table VIII.

                                      TABLE VIII                                  __________________________________________________________________________             Curing                                                                            Boric                                                                     Agent                                                                             Acid                                                                              Catalyst                                                                           Gel Time (sec)                                          Example                                                                            Resin                                                                             phr phr phr  150° C.                                                                    160° C.                                                                    170° C.                                                                    Tg °C.                               __________________________________________________________________________    20   A   3   0.33                                                                              0.2  1086                                                                              484 303 134                                         21   A   3   0.33                                                                              0.3  532 340 212 148                                         22   A   3   0.33                                                                              0.4  353 232 152 148                                         23   A   3   0.33                                                                              0.5  250 163 100 153                                         24   A   2.7 0.437                                                                             0.3  662 454 278 137                                         25   A   2.7 0.437                                                                             0.4  445 319 203 149                                         26   A   2.7 0.437                                                                             0.5  288 230 141 146                                         27   A   2.7 0.437                                                                             0.6  221 170  91 153                                         28   A   2.7 0.524                                                                             0.3  751 508 339 147                                         29   A   2.7 0.524                                                                             0.4  528 390 256 147                                         30   A   2.7 0.524                                                                             0.5  386 274 164 148                                         31   A   2.7 0.524                                                                             0.6  283 200 132 158                                         32   B   2.7 0.65                                                                              0.6  319 --  158  153**                                      33   B   2.7 0.75                                                                              0.65 292 --  156  157**                                      34   B   2.7 0.7 0.65 286 --  142  149**                                      35   C   2.7 0.65                                                                              0.6  347 --  165  156**                                      36   C   2.7 0.75                                                                              0.65 313 --  155  157**                                      37   C   2.7 0.7 0.65 293 --  148  152**                                      38   C   2.7 0.8 0.8  205 162 100 154                                         39   C   3   0.25                                                                              0.3  342 234 165 147                                         40   C   3   0.25                                                                              0.4  233 163  98 144                                         __________________________________________________________________________     **60 min resident time at 170° C.                                 

EXAMPLE 41

A varnish comprising 125 parts of resin B, 37.33 parts of adicyandiamide (7.5%) in monomethyl ether of propylene glycol (69.9%) anddimethylformamide (22.6%); 2.5 parts of boric acid (20%) in methanol;and 1.1 parts of 2 methyl imidazole (50%) in methanol is prepared, asdescribed in Example 1. The Gel time of the varnish is measured at 150°and 170° C. periodically over a eleven day period. The results arecompiled in Table IX.

                  TABLE IX                                                        ______________________________________                                        Varnish Storage Stability at Room Temperature                                 No. of Days                                                                              0        3      4      5    11                                     ______________________________________                                        Gel Time at                                                                              386      381    --     387  386                                    150° C. (sec)                                                          Gel Time at                                                                              164      173    171    173  172                                    170° C. (sec)                                                          ______________________________________                                    

EXAMPLES 41 to 47

Varnishes are prepared by blending a 125 parts of a brominated epoxyresin having a bromine content of 20% and an epoxy equivalent weight of418, prepared from the diglycidyl ether of bisphenol A andtetrabromobisphenol A, dissolved in methylethylketone (80% solids); 40parts of a solution of dicyandiamide (7.5%) in monomethyl ether ofpropylene glycol (69.9%) and dimethylformamide (22.6%), (3 phr); acatalyst (pyridine or triphenyl phosphine) and in examples 43 and 46boric acid. The relative amounts of catalyst and boric acid are compiledin Table VIII. The gel time is measured. The results are compiled inTable X.

                  TABLE X                                                         ______________________________________                                                                           Gel Time at                                Example Catalyst  phr    Boric Acid phr                                                                          170° C. (sec)                       ______________________________________                                        41*     Pyridine  0.2    --        361                                        42*     Pyridine  0.4    --        308                                        43      Pyridine  1.1    1         319                                        44*     Pyridine  1.5    --        223                                        45*     Triphenyl 0.2    --        395                                                phosphine                                                             46      Triphenyl 1.5    1         354                                                phosphine                                                             47*     Triphenyl 1.5    --        202                                                phosphine                                                             ______________________________________                                         *Not an example of the invention                                         

EXAMPLES 48 TO 89

Varnishes with different levels of catalyst, curing agent, and inhibitorare prepared by the following procedure. The stroke cure rate, gel time,and the glass transition temperature are determined. The amounts ofcomponents, gel times and glass transition temperatures are compiled inTable XI.

A 20% solution of boric acid in methanol is added to brominated epoxyresin prepared by advancing the diglycidyl ether of bisphenol A withtetrabromobisphenol A to 4.7 to 5.0% epoxide, then back blending withdiglycidyl ether of bisphenol A to a 9.8 to 10.1% epoxide, the finalproduct is diluted to 80% solids in acetone. The mixture is stirred tillhomogeneous, then allowed to stand for different time intervals atambient temperature. To this solution is added a 10% solution ofdicyandiamide (10% by weight dicyandiamide in a 50/50 mixture ofdimethyl formamide and monomethyl ether of propylene glycol stirred tillhomogeneous), then a 40% solution of 2-methyl imidazole in methanol isadded and stirred till homogeneous. This solution is then allowed tostand at ambient temperature 2 hours before gel times or cured productsare made.

Gel time is determined via a stroke-cure procedure at 175° C. Glasstransition temperatures were determined via differential scanningcalorimetry (DSC) at a heating rate of 10° C. per minute, using themidpoint of the transition.

                  TABLE XI                                                        ______________________________________                                              Boric   2-methyl Dicyandi                                                                             Gel          Stand                              Exam  acid    imidazole                                                                              amide  Time         time                               ple   (phr)   (phr)    (phr)  (sec) Tg (°C.)                                                                      (hrs)                              ______________________________________                                        48    0.4     0.35     2.5     246* 146    24                                 49    0.4     0.35     3       246* 145    24                                 50    0.4     0.75     3         87.5*                                                                            135    24                                 51    0.4     0.75     2.5       86.5*                                                                            140    24                                 52    0.8     0.75     2.5     186* 143    24                                 53    0.8     0.75     3        181.5                                                                             142    24                                 54    0.8     0.35     3      288   148    24                                 55    0.8     0.35     2.5      349.5                                                                             154    24                                 56    0.6     0.55     2.75     189.5                                                                             149    24                                 57    0.6     0.55     2.75   197   148    24                                 58    0.6     0.55     2.75   190   140    0                                  59    0.6     0.55     2.75   214   144    1.5                                60    0.6     0.55     2.75   232   149    24                                 61    0.6     0.55     2.75   232   150    12                                 62    1.01    0.55     2.75   278   156    1.5                                63    1.51    0.55     2.75   270   158    1.5                                64    1.01    0.55     2.75   270   157    24                                 65    1.51    0.55     2.75   268   161    24                                 66    1.01    0.55     2.75   273   154    12                                 67    1.51    0.55     2.75   270   --     12                                 68    1.01    0.35     2.5    --    157    15                                 69    1.51    0.35     2.5    --    162    15                                 70    0.6     0.55     2.75   230   153    0                                  71    0.6     0.55     2.75   205   153    1                                  72    0.6     0.55     2.75   199   146    2                                  73    0.6     0.55     2.75   201   149    3                                  74    0.6     0.55     2.75   202   --     6                                  75    0.2     0.175    2      353   140    2                                  76    0.2     0.175    2.5    349   147    12-16                              77    0.2     0.35     2.5    140   138    12-16                              78    0.2     0.35     2      162   142    12-16                              79    0.4     0.35     2      298   151    12-16                              80    0.4     0.35     2.5    260   149    12-16                              81    0.4     0.175    2.5    451   150    12-16                              82    0.4     0.175    2      571   140    12-16                              83    0.3     0.2625   2.25   290   --     12-16                              84    0.3     0.2625   2.25   284   149    12-16                              85    0.8     0.175    2      583   148    12-16                              86    0.8     0.175    2.5    485   149    12-16                              87    0.8     0.35     2.5    360   153    12-16                              88    0.8     0.35     2      433   153    12-16                              89    1.4     0.263    2.25   232   154    12-16                              90    1.4     0.263    2.25   230   155    12-16                              ______________________________________                                         *Stroke times for Examples 48-58 are the average of two samples.         

We claim:
 1. An epoxy resin composition comprising(A) a polyepoxide; (B)a cure inhibitor consisting essentially of boric acid in an amount fromabout 0.01 to about 2.0 parts per 100 parts of polyepoxide; (C)optionally an epoxy resin catalyst; and (D) optionally an epoxy resincuring agent which does not contain phenolic hydroxyl moieties.
 2. Anepoxy resin composition of claim 1 which further comprises a curingagent which does not contain phenolic hydroxyl moieties.
 3. The epoxyresin composition of claim 2 which further comprises an epoxy resincatalyst comprising an amine, heterocyclic nitrogen containingphosphine, sulfide, ammonium, phosphonium or sulfonium containingcompound capable of accelerating the cure of the polyepoxide with acuring agent.
 4. The epoxy resin composition of claim 3 wherein thecatalyst is an amine or heterocyclic nitrogen containing compound. 5.The epoxy resin composition of claim 4 wherein the catalyst is atertiary amine containing compound or a heterocyclic amine.
 6. The epoxyresin composition of claim 5 wherein the catalyst is 2-methyl imidazole;2-ethyl, 4-methyl imidazole or 2-phenyl imidazole.
 7. An epoxy resincomposition of claim 5 wherein between about 0.01 to 2.0 parts oftertiary amine or heterocyclic amine containing compound per hundredparts of resin are present.